# std::reverse

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class BidirIt > void reverse( BidirIt first, BidirIt last ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class BidirIt > void reverse( ExecutionPolicy&& policy, BidirIt first, BidirIt last ); | (2) | (since C++17)

1) Reverses the order of the elements in the range `[`first`,` last`)`.
Behaves as if applying std::iter_swap to every pair of iterators first + i and (last - i) - 1 for each integer i in `[`​0​`,` std::distance(first, last) / 2`)`.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If *first is not Swappable(until C++20)`BidirIt` is not ValueSwappable(since C++20), the behavior is undefined.

### Parameters

first, last | - | the range of elements to reverse
policy | - | the execution policy to use
Type requirements
- `BidirIt` must meet the requirements of LegacyBidirectionalIterator.

### Complexity

Exactly std::distance(first, last) / 2 swaps.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

See also the implementations in libstdc++, libc++, and MSVC STL.

    template<class BidirIt>
    constexpr // since C++20
    void reverse(BidirIt first, BidirIt last)
    {
        using iter_cat = typename std::iterator_traits<BidirIt>::iterator_category;

        // Tag dispatch, e.g. calling reverse_impl(first, last, iter_cat()),
        // can be used in C++14 and earlier modes.
        if constexpr (std::is_base_of_v<std::random_access_iterator_tag, iter_cat>)
        {
            if (first == last)
                return;

            for (--last; first < last; (void)++first, --last)
                std::iter_swap(first, last);
        }
        else
            while (first != last && first != --last)
                std::iter_swap(first++, last);
    }

### Notes

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type satisfies LegacyContiguousIterator and swapping its value type calls neither non-trivial special member function nor ADL-found `swap`.

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <vector>

    void println(auto rem, auto const& v)
    {
        for (std::cout << rem; auto e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    int main()
    {
        std::vector<int> v {1, 2, 3};
        std::reverse(v.begin(), v.end());
        println("after reverse, v = ", v);

        int a[] = {4, 5, 6, 7};
        std::reverse(std::begin(a), std::end(a));
        println("after reverse, a = ", a);
    }

Output:

    after reverse, v = 3 2 1
    after reverse, a = 7 6 5 4

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 223 | C++98 | std::swap was applied to each pair of iterators | applies std::iter_swap instead
LWG 2039 | C++98 | std::iter_swap was also applied when i equals std::distance(first, last) / 2 | not applied

### See also

reverse_copy | creates a copy of a range that is reversed (function template)
ranges::reverse (C++20) | reverses the order of elements in a range (algorithm function object)

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